Unfortunately the previous attempt to fix the dir seek system didn't
really work. Using a packed mid/rid integer for the offset is tempting,
but since mid/rid can change with any metadata id change in the
filesystem, dir tell offsets would become invalidated if you modified
files in unrelated directories, which isn't great and likely to catch
users by surprise.
This solution builds on the previous dir offset design, which tracks the
dstart-relative position independently from the current mid/rid in our
directory. To update this correctly when there are unrelated changes to
the filesystem, we need to know if metadata id changes are in the range
between our directories dstart and current mid/rid. This in turn means
we need to track our dstart. So our opened directories need three
separate pointers we need to update on every mdir commit:
dir->pos
|
.-------+-------.
a b c d e f g h i j k l m n o p
^ ^
| |
dir->dstart dir->mdir
This has quite a few moving parts, which I was hoping to avoid.
Fortunately we don't need a second mdir, so the RAM cost is pretty
small.
We can also drop dir->did, since the dstart mid/rid render it redundant,
which is interesting.
Christopher Haster
4cf5509c91